The present invention relates to a cellular code division multiple access (CDMA) communication system, and, particularly, it is assumed that packet data is transmitted using an enhanced uplink dedicated transport channel through a plurality of carriers.
A universal mobile telecommunication service (UMTS) system that is a third generation mobile communication system which is based on a global system for mobile communications (GSM) and general packet radio services (GPRS) that are a European mobile communication system and uses CDMA (code division multiple access) provide consistent services through which mobile phone or computer users can transmit packet-based texts, digitalized voice, video and multimedia data at a high speed faster than 2 Mbps anywhere in the world.
Particularly, the UMTS system uses a transmission channel which is called an enhanced uplink dedicated channel (hereinafter, EUDCH or E-DCH) in order to improve packet transmission performance in an uplink (UL) communication, i.e., a reverse direction communication from user equipment (UE) to base station (BS, node B). The E-DCH supports such technologies as adaptive modulation and coding (AMC), hybrid automatic retransmission request (HARM), base station control scheduling, and shorter transmission time interval (TTI) size in order to support stable, high-speed data transmission.
AMC is a technology that increases use efficiency of resources by determining the modulation method and coding method of data channel depending on the channel state between the base station and the terminal. Combination of the modulation method and the coding method is called MCS (modulation and coding scheme), and it is possible to define various MCS levels according to supportable modulation methods and coding methods. AMC adaptively determines the MCS level according to the channel state between the terminal and the base station, thereby increasing use efficiency of resources.
HARQ refers to a technique that retransmits a packet in order to compensate for an error packet when there is an error in an initially transmitted data packet. The composite retransmission technique can be divided into the chase combining (CC) technique which retransmits packets having format which is identical with the initially transmitted format when an error occurs and the incremental redundancy (IR) technique which retransmits packets having format which is different from the initially transmitted format when an error occurs.
Base station control scheduling refers to a method in which the base station determines whether to transmit uplink data and the upper limit of the possible data rate when transmitting data using E-DCH, and if the determined information is transmitted to the terminal as a scheduling command, the terminal determines the data transmission rate of possible uplink E-DCH with reference to the scheduling command and transmits the data.
Shorter TTI size reduces retransmission delay time by allowing TTI smaller than 10 ms, which is the minimum TTI of the current Rel5, thereby allowing high system throughput. A system or a service that uses such an E-DCH is called HSUPA (high speed packet access).
FIG. 1 illustrates uplink packet transmission through E-DCH in a wireless communication system. Here, reference numeral 100 refers to a base station that supports E-DCH, i.e., node B (hereinafter, “base station” and “node B” refer to the same), and reference numerals 101, 102, 103 and 104 refer to terminals that are using E-DCH. As illustrated, the terminals 101 to 104 transmit data to a base station 100 through E-DCH 111, 112, 113 and 114, respectively.
The base station 100 informs whether it is possible to transmit EUDCH data for each terminal by utilizing information of the data buffer state, requested data transmission rate or channel situation information, or performs scheduling of adjusting EUDCH data transmission rate. Scheduling is performed in a manner that allocates low data transmission rate to terminals (e.g., 103 or 104) which are located far away from the base station and allocates high data transmission rate to terminals (e.g., 101 or 102) which are located close to the base station while keeping the measured noise rise or RoT (rise over thermal) value of the base station under the target value in order to increase the performance of the overall system. The terminals 101 to 104 determine the allowed maximum data transmission rate of E-DCH data according to the scheduling information, determine the transmission rate of E-DCH data within the allowed maximum data transmission rate, and transmit E-DCH data.
In the uplink, uplink signals transmitted by different terminals do not keep synchronization between the signals, so they are not orthogonal and operate as interference to each other. Hence, the greater the amount of uplink signals received by the base station becomes, the greater the amount of interference to the uplink signals of a certain terminal becomes, thereby lowering the receiving performance. In order to overcome this problem, it is possible to increase the uplink transmission power of the certain terminal, but again it will operate as interference to other uplink signals, thereby lowering receiving performance. Hence, the entire power of the uplink signal, which can be received while the base station secures receiving power, is limited. RoT (Rise Over Thermal) represents wireless resources the base station uses in the uplink, which is defined in the following equation 1.MathFigure 1ROT=I_0/N_0  [Math.1]
Here, I_0 is power spectral density for the entire receiving band of a base station, and indicates the amount of the entire uplink signals received by the base station. N_0 is thermal noise power spectral density of the base station. Hence, the allowed maximum ROT is the entire wireless resources that the base station can use in the uplink.
The entire ROT of a base station is represented by the sum of interference between cells, voice traffic and E-DCH traffic. If the base station control scheduling is used, the phenomenon that several terminals simultaneously transmit packets at a high data transmission rate can be prevented, so that the receiving ROT can be kept within the target ROT, thereby always securing receiving performance. That is, the base station control scheduling prevents the receiving ROT from being increased to more than the target ROT by not allowing high data transmission rate to other terminals in case high data transmission rate is allowed to a certain terminal.
FIG. 2 is a flowchart illustrating a typical transmission and reception procedure through E-DCH.
Referring to FIG. 2, a base station and a terminal set E-DCH (202). The setting process at step 202 includes a transmission process of messages through a dedicated transport channel. When E-DCH is set, the terminal informs the base station of scheduling information (204). The scheduling information may be terminal transmission power information that indicates reverse direction channel information, extra power information that the terminal can transmit, and the amount of data to be transmitted, which has been piled up in the buffer of the terminal, or the like.
The base station which received scheduling information from a plurality of terminals that are communicating monitors the scheduling information of the plurality of terminals in order to perform scheduling on data transmission of each terminal (206). Specifically, the base station determines to allow reverse direction packet transmission to the terminal, and transmits a scheduling command to the terminal (208).
The scheduling command can direct the terminal to increase/maintain/decrease the allowed maximum data transmission rate through RG (relative grant) command. Alternatively, the scheduling command can direct through AG (absolute grant) command including the allowed maximum data transmission rate and transmission-allowed timing. The downlink physical channel that transmits the RG command is called E-RGCH (E-DCH Relative Grant Channel), and the downlink physical channel that transmits the AG command is called E-AGCH (E-DCH Absolute Grant Channel.
The terminal determines the transport format (TF) of E-DCH to be transmitted in a reverse direction using the scheduling command (210), and transmits reverse direction (UL) packet data (212) and transmits the TF information to the base station at the same time (214) through E-DCH. Here, the TF information includes a transport format resource indicator (TFRI) which indicates resource information that is necessary in de-modulating E-DCH. Here, at step 214, the terminal selects the MCS level in consideration of the allowed maximum data transmission rate allocated from the base station and the channel state, and transmits the reverse direction packet data using the MCS level.
The base station determines whether there is an error in the TF information or the packet data (216). As a result of the determination at step 216, if an error is found, the base station transmits NACK (negative acknowledge) to the terminal (218). On the other hand, as a result of the determination at step 216, if an error is not found, the base station transmits ACK (acknowledge) to the terminal through ACK/NACK channel (218). In case ACK information is transmitted, the packet data transmission is completed, and the terminal transmits new user data through E-DCH, however, in case NACK information is transmitted, the terminal retransmits packet data of the same contents through E-DCH. Here, the downlink physical channel through which the ACK/NACK is transmitted is called E-HICH (E-DCH HARQ Indicator Channel).